1. Field of the Invention
The present invention relates to a unit pixel of a Complementary Metal Oxide Semiconductor (CMOS) image sensor, more particularly, which provides a current compensating for a dark current to a photo diode to prevent a drive transistor from being quickly saturated by the dark current of the photo diode, thereby increasing a dynamic range.
2. Description of the Related Art
In general, each part of objects present in the natural world differs in brightness and wavelengths of light. An image sensor is a device that converts different brightness and wavelengths of the objects into an electrical value of a signal processable level, using photo-reactive properties of semiconductors.
Typically, the image sensor is used at a per-pixel level. A plurality of image sensors are aligned on a line of certain standard to produce a pixel array. Then images of a certain standard are picked up via the pixel array.
The aforesaid image sensor includes a photo-reactive semiconductor device and a plurality of transistors for outputting an electrical change of the semiconductor device as an electrical signal of a certain level.
FIG. 1 is a circuit diagram illustrating a unit pixel of a general CMOS image sensor according to the prior art. Referring to FIG. 1, the CMOS image sensor includes a photo diode PD for changing a capacity value in response to light, a reset transistor Q1 for resetting the photo diode PD to detect a next signal, a drive transistor Q2 for acting as a source follower via an electrical signal stored in the photo diode PD and a select transistor Q3 for selecting an output of a detected value.
That is, if the reset transistor Q1 stays on for a predetermined duration in response to a reset signal Rx, charges remaining in the photo diode PD are released and the photo diode PD is emptied so that current is stored in the photo diode PD at an amount proportionate to the capacity value corresponding to light. In addition, the transistor Q2 amplifies a voltage of the photo diode PD into the electrical signal (output voltage) within a set range to output. The output voltage from the drive transistor Q2 is outputted in the addressing order of a pixel array if the select transistor Q3 is turned on.
In this conventional CMOS image sensor, the photo diode PD, even though not exposed to light at all, generates a dark current which is a leakage current. That is, the dark current causes the drive transistor Q2 to generate the output voltage, even if not receiving light at all.
Such a dark current is supplied to the drive transistor Q2, thereby shortening a saturation time of the driver transistor Q2. In a further explanation, the dark current is combined with current generated when the photo diode PD receives light, thereby shortening a saturation time. Disadvantageously, the dark current generated in the photo diode PD reduces a driving range of the CMOS image sensor.
Especially, the dark current, which is considerably sensitive to temperature, substantially doubles or more with 10□ increase in its ambient temperature. As a result, the dark current further reduces a driving range of the CMOS image sensor.
In a conventional method to overcome the dark current-induced problem, an average value of the dark current generated in a dark pixel was calculated to compensate for an output of a non-dark pixel. However, according to such a conventional technology, the dark current generated in each pixel is not decreased but subtraction is performed externally based on a formula so that the unit pixel, if saturated, is inevitably degraded.